Activated carbon is made from carbon materials such as carbonized coconut shell, petroleum coke or coal coke that is activated to have a porous structure. The activated carbon that is porous and thus has a large surface area has been widely used as electrode material for double layer capacitors and lithium secondary batteries. In particular, in order to increase the energy density, i.e., capacitance in an electric double layer capacitor used in a hybrid car or the like, an activated carbon with effectively formed fine pores, a high crystallinity and a large surface area has been demanded to be used as an electrode material for the capacitor.
For industrial production of such activated carbon with effectively formed fine pores that can be used as an electrode material of an electric double layer capacitor, an activation method has been generally used, in which a carbon material such as petroleum coke and an alkali metal compound such as potassium hydroxide are heated at a temperature of 600 to 1200° C. in an inert gas atmosphere to allow the alkali metal to ingress between and react with graphite crystal layers. In this activation, the alkali metal enters a layered structure wherein condensated polycyclic hydrocarbons are layered, thereby forming fine pores.
The activated carbon produced by the alkali activation is required to have a relatively large surface area, a small average particle diameter, and a uniform particle size, and contain no bulky particles for the production of an electric double layer capacitor electrode.
In recent years in particular, an electric double layer capacitor used for hybrid cars and electric cars is required to be excellent not only in energy density but also output characteristics.
Conventionally, for the production of an electric double layer capacitor electrode, activated carbon is ground with a ball mill so as to make the particle size uniform thereby producing an activated carbon with a BET specific surface area of 1300 m2/g or greater and 2200 m2/g or smaller and an average diameter of 1 μm or greater and 7 μm or smaller (Patent Document 1). In Patent Document 2, an activated carbon with an average diameter of 100 nm to 10 μm is produced by a ball mill grinding method.
Whereas, it is reported in Patent Document 3 that an activated carbon with a small diameter is used to enhance output characteristics. However, this is not sufficient for recent large electric current charge and discharge applications.                (1) Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2000-182904        (2) Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2006-324183        (3) Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2003-077458        